1. Field of the Invention
The present invention relates to a method of scanning a charged beam to draw a micropattern of an LSI or the like on a target and, more particularly, to a method of automatically correcting astigmatism of a variable shaped beam.
2. Description of the Related Art
An electron beam lithographic apparatus using a variable shaped electron beam is known as a conventional apparatus for drawing a desired pattern on a target such as a semiconductor wafer. In this electron beam lithographic apparatus, a beam emitted from an electron gun and passing through an aperture is focused by various electron lenses, and the focused beam is radiated onto the target. Since any electron lens has astigmatism, the shaped beam also has astigmatism. Astigmatism in the beam causes degradation of drawing precision, and this astigmatism must be eliminated. For this purpose, the following method is employed. A mark on a target is scanned with a shaped beam to obtain a back-scattered or secondary electron signal, and the resolution of the shaped beam is measured by using this signal. The measured resolution is then utilized to correct astigmatism by astigmatism correction coils arranged near an objective lens.
This conventional method, however, poses the following problem. A conventional method of correcting astigmatism of a shaped beam in an electronic beam lithographic apparatus is performed in a direction of 0.degree. parallel to a given side of a rectangular beam and a direction of 90.degree. perpendicular to the given side. Scanning in only these directions makes it very difficult to measure a resolution at a corner portion of the rectangular beam. For example, no practical method for astigmatism correction in directions of, e.g., 45.degree. and 135.degree. has yet been proposed to date. It is very difficult to obtain a high-precision shaped pattern. Even in astigmatism correction in directions of 0.degree. and 90.degree., when astigmatism and a focus are corrected by measuring a beam resolution from a back-scattered or secondary electron signal obtained upon scanning of a mark on a target with a shaped beam, resolution of the beam cannot be precisely measured due to the tendency to get influences such as noise in measurement of the resolution of the beam, and sufficient correction precision cannot be obtained. When complicated processing of a beam wave is performed to improve precision, a processing time is undesirably prolonged.
In the conventional charged beam lithographic method using a variable shaped beam, astigmatism is corrected on the basis of the beam resolution obtained by scanning the mark with the shaped beam. According to this method, however, the beam resolution cannot be precisely measured due to the tendency to get influences such as noise in measurement of the resolution of the beam, and correction precision is undesirably degraded. When an improvement of correction precision is attempted, the processing speed is, in turn, decreased. It is difficult to easily correct astigmatism in a direction inclined from each side of the shaped beam, e.g., directions of 45.degree. and 135.degree..